Normally developed children show many diverse patterns of behaviour. By contrast, the behaviour of severely mentally retarded children is restricted mainly to primitive motor acts called stereotypes. Due to the severity of the mental retardation, these children are also markedly reduced in their susceptibility to environmental influences. Systematic observations of the pathological motor patterns of these children make it feasible to investigate possible endogenous mechanisms related to these primitive motor activities. The stereotyped motor behaviours of six severely mentally handicapped children were observed continuously for several days. Each child performed at least two forms of stereotyped behaviour. Using spectra of variance and cross-correlation analysis, the data were investigated with respect to underlying periodicities. A 2.5-hour cycle was consistently found in every stereotyped activity observed. Furthermore, stable phase relationships were revealed between the periodic variations of different stereotyped movements in one and the same child. Thus the temporal courses of hand and oral stereotypes were synchronous, whereas head and whole-body stereotypes were delayed by half a phase with respect to stereotyped hand waving. A possible relationship is discussed between the 2.5-hour periodicity of stereotyped behaviour and Kleitman's "Basic Rest Activity Cycle".